1. Field of the Invention
The present invention relates to a diagnosis system and method of an oxygen sensor for hybrid vehicles, and particularly to a diagnosis method of an oxygen sensor for hybrid vehicles w in maximum the generation of a harmful exhaust gas in case of an abnormal operation in such a way to monitor if an oxygen sensor operates normally.
2. Description of Related Art
Vehicles are often equipped with an oxygen sensor for meeting emission standards in various states and countries. In recent years, vehicles have begun to be equipped with a diagnosis function which allows the vehicle to diagnose an oxygen sensor during its active state to satisfy OBD (On Board Diagnostic) regulations as well.
In hybrid vehicles in particular, the above mentioned diagnosis function is performed under the control of a HCU (Hybrid Control Unit) which is configured to control the substantially every function which is executed within the vehicle and an EMS (Engine Management System) which is configured to control the engine. The decision to turn on or off the engine and injection is totally under control of the HCU. Thus, the EMS, which is in communication with the HCU, is turns on and off the engine in the vehicle based on control signals from the HCU.
In hybrid vehicles in particular, the following conditions when starting an oxygen sensor diagnosis function. When a diagnosis start request is received from the EMS (Engine Management System), the HCU (Hybrid Control Unit) starts controlling the engine to operate without stopping within a certain speed range for a certain amount of time in a fuel-cut state, so a required condition for the start of the diagnosis function can be satisfied. For example, the diagnosis request signal might consist of a long fuel-cut signal (5 sec/instance, for a total one time) and a short fuel-cut signal (3 sec/instance, total three times) to satisfy the OBD (On Board Diagnostic) regulation.
The conventional hybrid vehicle is generally configured to have the engine driven in a state that an engine clutch is actively engaged using a passive run start of an engine to satisfy the required condition for the diagnosis functions of an oxygen sensor. When there is an oxygen sensor diagnosis request from the EMS, the HCU determines if the engine is currently in a passive-run state. If the above mentioned condition is satisfied, the engine clutch is engaged, the injection is turned off and the fuel is cut. When the condition for an oxygen sensor diagnosis is satisfied, EMS starts performing the oxygen sensor diagnosis.
The conventional diagnosis method for an oxygen sensor of a hybrid vehicle, however, has the following problems. First, uneasiness on the part of the driver might somehow felt while the engine brake is being operated during a tip-out since the engine clutch remains engaged. This consequently causes operation of the vehicle feel different to the driver because a minor error occurs in the amount of the model computation-based engine friction torque. Second, the braking might also feel uneasiness also owing to the release of the engine clutch during regenerative braking as a driver steps on the brake. Therefore, this uneasy feeling on the part of the driver often because an error occurs in the regenerative braking operation owing to a transfer torque error in the engine clutch. Third, a delay occurs in the oxygen sensor diagnosis function owing to a start/release condition which occurs while the engine is being passively run. So, the diagnosis is completed after the vehicle has been running for about 20˜30 minutes.